This invention relates to a visible light titania photocatalyst, a method for making same, and processes for use of this catalyst.
The entire contents of all U.S. patents and published patent applications referred to below are herein incorporated by reference.
It has long been known that the semiconductor titania (titanium dioxide, TiO2) is also a photocatalyst that has been shown to be useful in a wide variety of photocatalytic applications, including but not limited to, generation of hydrogen from aqueous media, and disinfection and detoxification of gases, liquids, and surfaces. Ultraviolet illumination of titania produces positive charges, or holes, and negative charges that dissociate water molecules provided by even ambient humidity in air into hydroxy ions (OH−) that are adsorbed on the titania, and hydrogen ions. Hydroxyl radicals (OH•) are then formed when the hole accepts an electron from the adsorbed hydroxy ion. These highly reactive OH• radicals are powerful oxidizing agents, second only to fluorine and many times stronger than concentrated bleach. Hydrogen peroxide (H2O2) and oxygen radicals (O2−), also powerful oxidizers, are also formed. These agents have been shown to act together or separately to dissociate many organic molecules and other pollutants into harmless compounds, such that they can be used to remove contaminants from air and water, killing even drug-resistant bacteria and viruses on surfaces or in air and water, and reducing NOx and other pollutants in air. As a result, titania photocatalysts are found now in commercial applications ranging from self-cleaning films on windows to self-cleaning buildings built from titania-containing concrete to indoor air and surface disinfection when combined with artificial ultraviolet light sources.
For general background information relating to titania as well as carbon and nitrogen doped titania photocatalysts, including its uses and synthesis methods see:    1. M. R. Hoffmann, S. T. Martin, W. Choi, D. W. Bahnemann, “Environmental Applications of Semiconductor Photocatalysis,” Chem. Rev. 95, 69 (1995).    2. U. Diebold, “The Surface Science of Titanium Dioxide,” Surface Science Reports 48, 53 (2003).    3. J. Ryu, W. Choi, “Substrate-Specific Photocatalytic Activities of TiO2 and Multiactivity Test for Water Treatment Application,” Environ. Sci. Technol. 42, 294 (2008).    4. W. A. Jacoby, P. C. Maness, E. J. Wolfrum, D. M. Blake, J. A. Fennell, “Mineralization of Bacterial Cell Mass on a Photocatalytic Surface in Air,” Environ. Sci. Tech. 32, 2650 (1998).    5. O. Legrini, E. Oliveros, A. M. Braun, “Photochemical Processes for Water Treatment,” Chem. Rev. 93, 671 (1993).    6. A. Fujishima and K. Honda, Nature, 238, 37 (1972).    7. C. D. Valentin, G. Pacchioni, A. Selloni, “Theory of Carbon Doping of Titanium Dioxide,” Chem. Mater. 17, 6656 (2005).    8. Y. Choi, T. Umebayashi, S. Yamamoto, S. Tanaka, “Fabrication of TiO2 Photocatalysts by Oxidative Annealing of TiC,” J. Mater. Sci. Lett. 22, 1209 (2003).    9. Y. Choi, T. Umebayashi, M. Yoshikawa, “Farication and Characterization of C-doped Anatase TiO2 Photocatalysts,” J. Mater. Sci. 39, 1837 (2004).    10. P. Górska, A. Zaleska, E. Kowalska, T. Klimczuk, J. W. Sobczak, E. Skwarek, W. Janusz, J. Hupka, “TiO2 Photocatalytic in Vis and UV Light: The Influence of Calcination Temperature and Surface Properties,” App. Catal. B: Env. 84, 440 (2008).    11. T. Tachikawa, S. Tojo, K. Kawai, M. Endo, M. Fujitsuka, T. Ohno, K. Nishijima, Z. Miyamoto, T. Majima, “Photocatalytic Oxidation Reactivity of Holes in the Sulfer- and Carbon-Doped TiO2 Powders Studied by Time-Resolved Diffuse Reflectance Spectroscopy,” J. Phys. Chem. B 108, 19299 (2004).    12. T. Morikawa, R. Asahi, T. Ohwaki, K. Aoki, Y. Taga, “Band-Gap Narrowing of Titanium Dioxide by Nitrogen Doping,” Jpn. J. Appl. Phys 40, L561 (2001).    13. L. Wan, J. F. Li, J. Y. Feng, W. Sun, Z. Q. Mao, “Improved Optical Response and Photocatalysis for N-Doped Titanium Oxide (TiO2) Films Prepared by Oxidation of TiN,” Appl. Surf. Sci. 253, 4764 (2007).    14. J. Wang et al., “Origin of Photocatalytic Activity of Nitrogen-Doped TiO2 Nanobelts,” J. Am. Chem. Soc. 131, 12290 (2009).
However, titania photocatalyst powders in their present state have not lived up to their enormous potential because they do not use visible light efficiently or at all. The band gap of titania, whether in its anatase or its rutile form, exceeds 3.0 eV, so that it only absorbs in the ultra-violet portion of the electromagnetic spectrum. Because there is very little ultraviolet light present in sunlight at the surface of the earth (sunlight integrated over the 3 eV to 4 eV range is less than 6 mW per square cm, compared to the 100 mW per square cm total in visible sunlight) and even less or no ultraviolet light indoors, commercially available titania photocatalysts have limited effectiveness in sunlight, and for indoor use they require artificial ultraviolet light sources to work. The latter must be shielded from skin and eyes, create ozone pollution, and are expensive to operate because of their inefficient conversion of electricity to ultraviolet light.
Given the important uses of titania and the poor photocatalytic efficiency of titania in the absence of ultraviolet light, much effort has been devoted to reducing the band gap of titania in order to improve its photochemical efficiency. For example, U.S. Pat. No. 7,628,928 describes a method in which a stressed titania film is formed on a spherical substrate having a sufficiently small radius to cause stress in the titania film and thereby shift its band gap to support photocatalytic detoxification and disinfection in visible light. Similarly, U.S Published Application No. 2008/0299697 describes a process for producing a titania electrode comprising anatase having a bandgap lower than that of unstressed anatase. This process comprises subjecting titanium metal to an etchant, and then oxidizing at least part of the etched titanium to anatase by anodizing the titanium in an anodizing solution, and/or heating the titanium in an oxygen-containing atmosphere.
Another approach to reducing the band gap of titania is by doping, that is to say by introducing atoms of other elements into the titania crystals. For example, U.S. Pat. Nos. 7,096,692 and 7,749,621 describe a visible light photoactive coating produced by doping titania with one of more of the metals chromium, vanadium, manganese, copper, iron, magnesium, scandium, yttrium, niobium, molybdenum, ruthenium, tungsten, silver, lead, nickel and rhenium. U.S. Pat. Nos. 7,637,858 and 7,651,675 describe a process of producing a nitrogen-doped titanium oxide for a photocatalyst having light absorption in the visible light region. The doped titanium oxide is prepared by preparing a titania/organic substance composite including an organic ligand coordinated to flaky titania and forming a layered structure; immersing this titania/organic substance composite in aqueous ammonia; drying the resulting composite; and, after the drying step, heating the composite at a temperature of 400 to 500° C., whereby nitrogen is doped into titania by thermal decomposition of the ammonium and, in addition, titania is crystallized to an anatase form. U.S. Pat. No. 7,141,125 describes a peroxo-modified titania intended for use in photocatalysis. U.S. Pat. No. 5,242,880 describes anatase titania provided with sodium, potassium, calcium, magnesium, barium, zinc, or magnesium salts of sulfuric or phosphoric acid, and stated to be useful in the pigmentation of oxidizable polymers. U.S. Pat. No. 6,703,438 describes an electroconductive plate-like titania containing at least 10% by weight of titanium nitride and at least 0.1% by weight of carbon, which is claimed to provide electroconductive parts which are higher and more uniform in electroconductivity. Finally, U.S. Published Application No. 2010/0062928 describes a method for producing titania doped with carbon atoms and nitrogen atoms (and optionally metal atoms). This doped titania is produced by dispersing or dissolving a basic polymer having amino groups in aqueous medium; obtaining a layered structure composite of polymer/titania with the basic polymer inserted among titania by mixing the aqueous dispersion or solution and a water-soluble titanium compound in an aqueous medium, and producing a hydrolytic reaction at a temperature of 50° C. or less; and burning with heat the layered structure composite.
Multiple publications report the synthesis of nitrogen and carbon-doped titania claiming enhanced and visible light photocatalytic activity. Methods include the low temperature (350-750° C.) calcination of titania precipitates from sol-gel processes both with and without additional carbon or nitrogen containing precursors. A single step low temperature calcination (400° C.) of titania powder mixed with carbon containing chemicals such as urea and thiourea have also been reported to dope titania with carbonate species. The oxidation of titanium carbide powder has been studied over a range of temperatures (350-800° C.) and has shown a modest enhancement due to carbon doping when oxidized at the lower end of this temperature range. Similar work has been performed studying the oxidation of TiN powders and films with reports of nitrogen doping percentages typically close to 0.1%. These published results achieve only modest increases in visible light photocatalytic activity or even report substantial reductions in overall photocatalytic activity as a result of the carbon or nitrogen doping.
It has now been found that a carbon and/or nitrogen-doped titania having high photocatalytic activity in visible light can be produced by a simple two-step process starting from commercial grades of titania and carbon powders and nitrogen gas, and the present invention relates to this process, to the doped titania thus obtained, and to processes for use of this doped titania.